Regulated and non-regulated pump drives are known from the state of the art, whereby regulated pump drives are more efficient. For example, a regulated pump system can be configured with a variable speed and can essentially comprise a drive unit consisting of a frequency converter and control electronics, a standard electric motor or an electric servomotor as well as a hydraulic pump. In this context, the delivery rate of the hydraulic pump is proportional to the input speed of the electric motor. During operation, the machine control unit transmits the target values of the pressure/volumetric flow to a controller. The prevailing system pressure is detected by a pressure measuring means and is likewise transmitted to the controller. On the basis of the control deviation, the controller calculates the necessary motor speed and adjusts it to the delivery rate and pressure in accordance with the applicable system requirements.
Radial piston pumps, for example, lend themselves as the hydraulic pump. The requisite drive torque is transmitted by a shaft via a coupler to a cylinder star that is mounted on a control journal. Pistons arranged radially in the cylinder star are supported, for example, via sliding blocks, on a stroke ring. The piston and the sliding block are joined together, for instance, by means of a ball-and-socket joint. The sliding blocks pass through the stroke ring and, during operation, they are pressed against the stroke ring by centrifugal force and by oil pressure. When the cylinder star rotates, the pistons execute a stroke movement due to the eccentric position of the stroke ring. The eccentricity is changed by displacement pistons that are actuated by a pilot valve. Changing the eccentricity influences the magnitude of the stroke, whereby the delivered fluid volume results from the stroke and the speed.
In order to influence the behavior of the pressure control loop, the state variable pertaining to the pressure change or to the delivery rate is needed for the control loop. The delivery rate and the pressure change are of the same order in systems having hydraulic capacities. The delivery rate is directly proportional to a given speed in speed-controlled piston pumps, whereas it is directly proportional to the pivoting angle or to the position of the stroke ring in displacement pumps, and it is directly proportional to the position of the valve slide in the case of valve control units. The delivery rate can be measured directly without delay. The pressure change can be obtained through differentiation of the pressure signal. The use of the delivery rate in the feedback improves the dynamic behavior of the control loop but, in the case of disturbance situations of the control loop in which fluid is consumed, it gives rise to control errors. Providing an additional integrator in the control loop has proven its worth as a measure for minimizing such control errors. As an alternative, the signal can be applied via an extremely low-frequency high-pass filter, which leads to a decoupling of the common mode portion.
Both of these measures detrimentally affect the dynamics of the disturbance characteristic. This drawback does not exist if the pressure change instead of the delivery rate is employed as the feedback. A problem in this context, however, is the detection of the pressure change. The pressure is measured and the pressure change is ascertained by means of differentiation of the pressure. In pump applications, the pressure signal is very noisy. Differentiation without low-pass filtering is only of limited usefulness in the control loop. The low-pass filtering often has to be carried out in the fundamental frequency range of the control loop. Due to this delay in the detection of the pressure change, the power of this state variable to influence the control dynamics cannot be fully utilized.